So I just installed a flowmaster cat back exhaust this past weekend, then reflashed my diablosport predator 93 tune. I noticed a noticable performance gain, but my gas mileage tanked.. Before the exhaust i was averaging 16.5 mpg with all in town driving, and now by driving very very nice I'm barely getting 15 mpg.

Seriously everyone I talked to said my mileage would improve with a tune and aftermarket exhaust, and CAI... Not the case with me! LOL.

I definitely agree with the above posters saying that it's prolly you or the way you drive that changed since the mods. It makes sense that with a new tune and better sounding exhaust you would give it more gas than normal.

A good test would be cruising on the highway, not in town where driving habits and traffic can greatly affect the mileage. How can you be sure that you didn't hit extra traffic or a few more lights? At least on the highway cruising at 65mph is the same everyday.

In town driving with so much stop and go is not a very accurate measure of mpgs.

Plus, more power = more fuel. You put in a performance tune. If you put in an mpg tune, I'd be concerned with the results. You put in their most aggressive tune which ups the fuel and spark for more power on premium fuel.

I'd bet your throttle response feels snappier. I think they tinker with the throttle reaction table to make the actual engine throttle react faster to the gas pedal moving. Making the throttle open faster will not improve the mpgs.

I've used that tune before and it does have a real nice throttle response to it, at the price of a little more fuel. Which if you are performance tuning, is the expected result.

Driving very very easy on it in the City Iam 13-14 ... it's not driving habits (when you watching the MPG Gauge and trying to see your Best Milage you can get) .. The OP states he was also trying to see his best milage by driving very easy on it. After you see what you can get then you drive like you really should be.